It has long been recognized that aluminum and its alloys require specific surface preparation processes to ensure successful electroplating. The main reason for this requirement is the high affinity of aluminum for oxygen and, as a result, a freshly clean aluminum surface will re-oxidize instantly. This oxide layer has been shown to negatively affect adhesion of plated metal coatings on aluminum if it is not properly controlled.
Pretreatment of aluminum and its alloys prior to electro or electroless plating generally involves several steps including: (1) cleaning, (2) etching, (3) desmutting, and (4) zincating.
Cleaning is performed to remove the various oils, greases, grits, soils and dirts that are present from material handling, corrosion protection, or other surface preparation. Cleaning can involve an array of chemistries and processes including aqueous chemistries, solvent degreasing, vapor degreasing, ultrasonic cleaning etc. Aqueous cleaning by immersion is the most popular process.
After cleaning, aluminum is treated in an alkaline or acidic solution to etch or roughen the surface and remove the heavy oxide layer. Oxide removal makes the surface more electrochemically active. The type of etching employed depends on the aluminum alloy, processing conditions and condition of the surface. An examination of the aluminum surface after etching typically reveals the presence of a loosely adherent film or smut on the surface, which negatively impacts the adhesion of subsequent plating to the aluminum. The composition of this smut depends on the alloy constituents in the aluminum, and generally contains metallic constituents. Thus after etch treatment, the substrate is subjected to a process (desmutting) to remove the smut layer.
A zincating process generally follows desmutting, where the aluminum is immersed in an acid or alkaline zinc bath to deposit a thin zinc-containing layer. The zincate layer controls and minimizes oxidation of the aluminum surface. A typical process sequence after desmutting includes water rinsing, zincating, chemical stripping of zincate layer, zincating the surface again, followed by electrolytic or electroless strike to put permanent metal layer on aluminum.
Experience has shown that desmutting is one of the most critical steps in the aluminum pretreatment process. Historically, nitric acid solutions, with acid concentrations of 25% to 70%, have performed well to desmut etched aluminum alloys. Not all smut is easily removed with nitric acid alone, thus often additions of other components are made to improve the effectiveness of the desmut. For instance, for aluminum alloys containing high concentrations of silicon (e.g., 356 A and 380 series cast alloys), additions of fluoride-containing compounds such as ammonium bifluoride or sodium fluoride, have been added such that fluoride ions are available to dissolve and remove silicon from the surface. Alternatively, a solution of nitric acid, sulfuric acid and a fluoride-containing salt has gained popularity over the years, because of its ability to chemically attack and remove a wider variety of metallic smuts.
While nitric acid has been very effective for desmutting etched aluminum, there has been increased resistance to its use because of safety and health concerns. For instance, development of toxic NOx fumes in nitric acid-containing baths has been of particular concern. To obviate this concern, there has been significant effort to develop and use non-nitric acid containing desmuts. One such approach has employed the use of chromic acid as the oxidant, again combined with sulfuric acid and a fluoride containing salt. This approach was successful for desmutting and avoids NOx concerns. However, the use of chromic acid brings with it toxicity concerns of its own.
An object of the present invention is to provide a composition that is free of acids such as nitric acid and chromic acid, thus eliminating the health, safety, and environmental concerns associated with these acids, but which is capable of being highly effective in its desmutting ability.